Factory casting, or precasting, of concrete panels usually takes place on a large steel platform (casting bed) in a precast yard. The sizes and shapes of the panels are determined by sideforms that are arranged on the platform, and concrete is poured into the space defined by the sideforms. When the concrete is dry the panels are transported for installation.
Alternatively, concrete panels may be poured on-site, or tilt-up. In this case the panels are cast either on concrete slabs or on transportable steel beds. Again sideforms are used to define the size and shape. The panels are subsequently lifted into position using a crane. Due to space constraints, site casting frequently involves pouring several panels one on top of another. After the lowermost panel is dry it is coated with a release agent, and the sideforms are moved up to define a new panel of the same size or smaller before a second pour. The crane lifts the panels one at a time from the stack and moves them into position.
Standard sideforms are available in the range of standard concrete panel thicknesses 125 mm, 150 mm, 175 mm, 180 mm and 200 mm.
It is virtually impossible to produce a crisp sharp edge on cast concrete, since concrete is made up of granular particles and the sharpness of the corner is governed by the size of the particles. A sharp edge would also highlight formwork that is not perfectly straight and true, and also such an edge would chip very easily. As a result all standard sideforms are shaped to place a 45° chamfer on the concrete edge so as to hide the error in trueness and also prevent damage to the concrete edge.
Folded steel plate sideforms are generally used for precasting, but not for tilt-up as they are too heavy. These sideforms are made from a steel plate that has 45° plays pressed into the top and bottom edges to form the chamfer. The plate is then welded to a steel angle, channel or square hollow section to give it strength and stability.
Aluminium sideforms are also used for both pre-cast and tilt-up applications. There are several types with locking channels at the top and bottom edges, or a keyhole or ‘V-Lock’ locking strip in the rear. Some of these have different angles of splay, but they are generally bulky and expensive to extrude.
When a non-standard angle is required, the only option, currently, is to use fillets cut from plywood or polystyrene foam. This is wasteful, slow, extremely labour intensive and does not produce a nice accurate finish.
Sideforms are secured to the casting bed by screw-fixing. In this case the screw holes require repair after every cast. An alternative is to bolt the sideform to the casting bed, or to a securing member, which is generally either an angle or magnet. The bolting is time consuming and the resulting structure is very heavy and difficult to manoeuvre. An alternative is to lock the securing member to the sideform using a square or ‘V’-groove channel in the rear of the sideform. This technique is prone to problems when excess concrete falls into the channels and sets there.
No casting bed or surface is perfectly true and flat, whether it is made from steel, concrete or any other material. Neither are the sideforms. Gaps between the bed and the sideforms result in bleeding of water and fine particles. The result is weak and crumbly patches in the panel that have to be repaired. The most common method of preventing concrete bleeding is to place a bead of silicone between the underside of the sideform and the casting bed. Although this is effective in sealing leaks, it gives rise to substantial costs in time and labour to scrape and grind the cured silicone residue from the casting beds and sideforms before they can be used again. It also causes wear and tear and damage to both surfaces.
After the concrete is poured an aluminium screed trowel is brought down onto the concrete surface at the correct height and moved back and forth to cut the surface down to the correct height. When the trowel initially cuts the concrete surface to the right level there is concrete residue left on the trowel. Some of this falls off the trowel onto the casting bed or support mechanisms, and some is left on the already cut concrete surface. When a hand or power trowel is then used to finish off the concrete surface these implements do not cut the concrete surface and tend to ride the areas where the concrete residue has been deposited, which results in these areas being slightly elevated.
There is also the cost in time, labour and productivity in cleaning the excess concrete spillage from the casting bed.